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Abstract

Optical Coherence Tomography (OCT) is an optical interferometric technique developed mainly for in vivo imaging of the eye and biological tissues. In this paper, we demonstrate the potential of OCT for non-invasive examination of museum paintings. Two en-face scanning OCT systems operating at 850 nm and 1300 nm were used to produce B-scan and C-scan images at typical working distances of 2 cm. The 3D images produced by the OCT systems show not only the structure of the varnish layer but also the paint layers and underdrawings (preparatory drawings under the paint layers). The highest ever resolution and dynamic range images of underdrawings are presented and for the first time it is possible to find out non-invasively on which layer the underdrawings were drawn.

C. R. T. Young and R. Hibberd, “The role of attachments in the degradation and strain distribution of canvas paintings,” in Traditions and Innovation: Advances in Conservation, (International Institute of Conservation Melbourne Congress October2000), pp212–220

D. Ambrosini and D. Paoletti, “Holographic and speckle methods for the analysis of panel paintings. Developments since the early 1970s,” Reviews in Conservation 5, 38–48 (2005).

Hibberd, R.

C. R. T. Young and R. Hibberd, “The role of attachments in the degradation and strain distribution of canvas paintings,” in Traditions and Innovation: Advances in Conservation, (International Institute of Conservation Melbourne Congress October2000), pp212–220

Yang, M.-L.

Young, C. R. T.

C. R. T. Young and R. Hibberd, “The role of attachments in the degradation and strain distribution of canvas paintings,” in Traditions and Innovation: Advances in Conservation, (International Institute of Conservation Melbourne Congress October2000), pp212–220

C. R. T. Young and R. Hibberd, “The role of attachments in the degradation and strain distribution of canvas paintings,” in Traditions and Innovation: Advances in Conservation, (International Institute of Conservation Melbourne Congress October2000), pp212–220

A 1300nm OCT image of a mastic varnish layer (on a piece of glass) dating from 1952; the varnish/air and varnish/glass interfaces are clearly delineated. The third (faint) interface is a ghost image or the result of multiple reflections between the varnish/air and varnish/glass interface. The vertical scale represents depth measured in air.

(a) Sample board of yellow ochre and smalt on a ground layer of chalk and rabbit skin glue. One third of the sample has three coats of dammar varnish applied, and another one third of the painting has 3 coats of mastic varnish applied, leaving the central part of the sample unvarnished; (b) 1300nm OCT image of a cross-section across the yellow ochre/smalt boundary (scan in the middle of the panel; c-d) 850nm OCT images of cross-sections on the yellow ochre/smalt boundary (scan along the bottom line segment) and in the smalt area (scan along the top line segment). The vertical scale represents depth measured in air.

(a) A 50-year-old test painting: Point A is covered with the original varnish, which is now yellowed; Point B has the old varnish removed and new varnish applied; Point C is unvarnished; Point D is the boundary between regions where there is just one layer of old varnish and regions where a layer of new varnish was applied on top of the old varnish. (b-d) 1300nm OCT image of Point B, C and D. (e) 850nm OCT cross-section at Point A; The vertical scale represents depth measured in air.

(a) An 18th century panel painting; (b) a cross-section image of a scan along the top line-segment marked on the painting; (c) A cross-section image of along the lower line-segment on the painting. Both images were obtained with 1300 nm system.

(a) Color images of two painted patch over underdrawings: the top patch has two layers of lead-tin-yellow paint over underdrawings drawn with a quill pen using an ink of bone black in gum; the bottom patch has a mixture of lead white, azurite, bone black painted over a black chalk underdrawing; (b) the corresponding near infrared Vidicon images; (c) the corresponding near infrared images taken with a InGaAs camera; (d) the corresponding 1300 nm OCT images taken at the depth of the underdrawings. The size of the images are ~ 1 cm by 1 cm.

(a) Color image of a painted panel: the lower part is painted with an imprimatura (a translucent paint layer) on top of the underdrawing which is painted on a preparatory ground layer, the upper half has an additional paint layer above the imprimatura; (b) average of the top four en-face images collected with the 1300 nm system, i.e. 0–100 μm; (c) average of the next four en-face images (100–200 μm); (d) average of the next seven en-face images (200–375μm); (e) average of the next seven images (375–550μm); f) B-scan image in the white area of the panel showing the underdrawing below the first layer of paint (imprimatura); g) B-scan image in the blue area of the panel showing the underdrawing below two layers of paint.